1. Field of the Invention
The present invention relates generally to particle accelerators. More particularly, embodiments of the present invention relate to systems and methods for tuning particle accelerators.
2. Description of the Related Art
Particle accelerators have been used for a number of years in various applications. For example, one common and important application is their use in medical radiation therapy devices. In this application, an electron gun is coupled to an input cavity of a linear accelerator. The electron gun provides a source of charged particles to the accelerator. The accelerator then accelerates the charged particles to produce an accelerated output beam of a desired energy for use in medical radiation therapy.
It is important to ensure that the beam output from a particle accelerator is generated efficiently and is of the desired energy. The energy and other characteristics of the beam are dependent upon the resonant frequency of the accelerator which in turn depends upon the shape and manufacture of the accelerator. The operating efficiency of a particle accelerator is optimized when the resonant frequency of the accelerator matches the frequency of the applied driving signal. Although the physical characteristics of the acceclerator needed to achieve the desired resonant frequency may be determined precisely, imperfections in the accelerator cavity structure may result from variations in the accelerator manufacturing process. These imperfections tend to detune the accelerator cavity structure. As a result, accelerators generally must be tuned before they are used for their intended application.
This tuning process is an iterative process that is sequentially performed for each cavity of a particle accelerator until each cavity has been tuned to a desired resonant frequency. Existing tuning processes first require that a cavity to be tuned be isolated from other cavities in the particle accelerator by shorting adjacent cavities. An input signal is then applied to the cavity under test and a resonant frequency of the cavity is measured. A tuning technician typically compares the measured resonant frequency with an expected resonant frequency to determine if the cavity is properly tuned. If the measured resonant frequency is different than the expected resonant frequency, the tuning technician physically deforms the cavity by hitting an exterior surface of the cavity with a hard object, such as a hammer. This process is repeated for each cavity until the particle accelerator is properly tuned. The assignee of the present invention, in co-pending, and commonly-assiged U.S. patent application Ser. No. 09/546,409, filed Apr. 10, 2000 for xe2x80x9cCOMPUTER-AIDED TUNING OF CHARGED PARTICLE ACCELERATORSxe2x80x9d (the contents of which are incorporated in their entirety herein for all purposes) has developed a way to increase the efficiency of tuning such devices with the assistance of computer automation.
Many existing particle accelerators use coupling cavities moved off the beam axis (xe2x80x9cside cavitiesxe2x80x9d) to provide coupling between primary cavities. Use of these side cavities can complicate the tuning of a particle accelerator. Currently, to tune a primary cavity, adjacent side cavities are decoupled from the primary cavity. The side cavity is typically decoupled (or taken out of resonance with the primary cavity) by placing the side cavity in a de-tuned condition. This condition presently requires use of access ports fabricated into each side cavity. These access ports can also complicate the manufacturing process, making it difficult to fabricate side cavities having desired microwave characteristics. The use of access ports also increases the cost of manufacturing side cavities.
Perhaps more importantly, however, the use of these access ports can result in decreased operating efficiency of the particle accelerator after tuning because the access ports must be sealed after the tuning process has been completed. These access ports are sealed by brazing or welding a metal cap onto the access port after tuning. The high temperatures required to cap the access port can deform the side cavity resulting in a change in the resonant frequency of the cavity. Because the access port is sealed, the side cavity (and thus the particle accelerator) cannot be retuned after sealing. As a result, the overall efficiency of the particle accelerator can be degraded.
Typical tuning methods measure the resonant frequencies of individual cavities by isolating adjacent cavities. In operation, however, operation of a particle accelerator involves the interaction of a number of adjacent cavities in the accelerator. Gu, et al., in xe2x80x9cA TUNING METHOD FOR SIDE COUPLED STANDING WAVE ACCELERATING TUBESxe2x80x9d, Nuclear Instruments and Methods of Physics Research (1987), 339-342, describe a manual tuning technique which measures three coupled modes (involving three cavities, the primary cavity and two side cavities) by resonating the two primary cavities adjacent to the primary cavity under test. While this allows tuning of an accelerator having side cavities formed without access ports, the multiple variables involved require many testing iterations to arrive at a tuned cavity. Further, tuning is complicated because the measured three modes depend heavily on the primary cavity to be tuned. Thus, a substantial number of iterations is needed to converge toward the target frequency.
It would be desirable to provide a tuning method and apparatus which reduces the number of variables affecting the tuning process. Further, it would be desirable to provide a tuning method and apparatus which reduces the amount of manual intervention required, while still allowing use of an accelerator having side cavities without access ports. It would also be desirable to provide a system and method that allows the particle accelerator to be repeatedly tuned after deployment and use.
To alleviate the problems inherent in the prior art, embodiments of the present invention provide a method, system and apparatus for tuning particle accelerators.
According to one embodiment of the present invention, a method, system, and apparatus for tuning a particle accelerator is provided which includes tuning side cavities while placing adjancent cavities in a de-tuned condition. A conductor is positioned such that a primary cavity under test is minimally excited, while adjacent side cavities are excited. Coupled modes are measured. The primary cavity is tuned based on the measured coupled modes. According to the invention, this tuning is accomplished without use of access ports to the interior of the side cavities.
According to one embodiment, the side cavities are tuned by placing adjacent cavities in a de-tuned condition and measuring a resonant frequency of the side cavity and deforming the side cavity if the measured resonant frequency is not equal to, or within an acceptable range of, an expected resonant frequency for the side cavity.
According to one embodiment, the coupled modes are measured by placing adjacent primary cavities in a de-tuned condition and then operating an analyzer to detect the coupled modes. According to one embodiment, the primary cavity is tuned by calculating a measured resonant frequency of the primary cavity using the measured coupled modes and the measured resonant frequency of the side cavities.
According to one embodiment, some or all of the tuning is performed under the control or direction of a computer. Means for tuning a particle accelerator are also provided.
The present invention is not limited to the disclosed preferred embodiments, however, as those skilled in the art can readily adapt the teachings of the present invention to create other embodiments and applications.